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Nanoscale metal paste for interconnect and method of useNanoscale metal paste for interconnect and method of use description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20090162557, Nanoscale metal paste for interconnect and method of use. Brief Patent Description - Full Patent Description - Patent Application Claims
This application is a continuation-in-part of U.S. patent application Ser. No. 10/589,399, filed on Aug. 14, 2006, published as U.S. Patent Application Publication No. 2007/0183920, which claims priority to the PCT Application No. PCT/US2005/004567, filed Feb. 14, 2005, which in turn claims priority to the U.S. Provisional Application Ser. No. 60/545,139, filed Feb. 18, 2004. The disclosure of these applications is hereby incorporated by reference herein. 1. Field of the Invention The present invention generally relates to materials used for interconnecting electronic devices and, particularly, devices which either generate high temperatures during use or devices which are used in high temperature applications. Furthermore, the invention is generally related to a fabrication method which reduces or eliminates the need for high pressure application during fabrication of an interconnection, such as during die attach. 2. Background Description All semiconductor chips have to be fastened or attached to a substrate to function in an electronic product. The state-of-the-art technology for interconnecting these chips typically uses a lead or lead-free solder alloy, or a conductive polymeric glue, such as an epoxy. However, these materials have poor thermal properties and do not dissipate the heat generated by the chips. They also have poor electrical properties and fail to effectively reduce loss of electrical power, and poor robustness for mechanical strength and reliability. Furthermore, because of the low melting temperatures of solder alloys and low decomposition temperatures of epoxies, these materials may not be generally suitable for allowing some chips, such as SiC or GaN chips, to function at high temperatures. Sintering of microscale metal powder paste is commonly used in hybrid electronic packages for producing electrical circuit patterns. However, the high processing temperatures (>600° C.) prevent its use in joining electronic components to substrates. The current practice is to use solder that is reflowed at temperatures low enough for the devices to withstand. The advantage of low melting temperatures becomes a liability for solder alloys because they cannot meet the requirements of high temperature operation or use in high temperature applications. Furthermore, solder materials have relatively poor electrical and thermal properties, and poor fatigue resistance, compared to other metals such as copper and silver, which detrimentally affect the performance of the whole electronic system. Pressure-assisted sintering using commercial silver metal paste to attach electronic components was discussed in Zhang et al., “Pressure-Assisted Low-Temperature Sintering of Silver Paste as an Alternative Die-Attach Solution to Solder Reflow”, IEEE Transactions on Electronics Packaging Manufacturing, vol. 25, no. 4, October, 2002 (pp 279-283); and Zhang et al., “Pressure-Assisted Low Temperature Sintering of Silver Paste as an Alternative Die-Attach Solution to Reflow”, The Fifth International IEEE Symposium on High Density Packaging and Component Failure Analysis in Electronics Manufacturing (HDP 2002) The metal powder in commercial silver metal paste typically has a particle size in the micrometer range. Because of the large particle size, a high sintering temperature is required (600° C. and up) under normal firing conditions. At reduced firing temperature, a large pressure is applied on the assembly to assist the sintering process. However, the application of pressure can be undesirable because of increased difficulty in manufacturing with a corresponding increase in the production cost. Applying pressure also increases the likelihood of damage to the device during processing. It has been discovered that by using very fine conductive metal and metal alloy particles on the order of 500 nm or less, and most preferably on the order of 100 nm or less (e.g., 1-100 nm), densified metallic interconnections can be established by relatively low temperature sintering with reduced or no pressure application being required. The materials of this invention can be applied and processed like a solder paste or epoxy (e.g., dispensing, stencil/screen printing, etc.). However, the thermal, electrical and mechanical properties of the joint formed with the fine powders and compositions thereof are far superior to those of traditional lead or lead free solders, epoxy materials, and even micron sized powders (sintered at low temperature). By using metal particles in the nanoscale range, it is possible to both reduce the bonding temperature (i.e., the sintering temperature in the context of the present invention), and to eliminate or reduce the need for high applied pressure. Without the need for high applied pressure, it is thus possible to make use of existing hybrid microelectronics processing techniques and fabrication equipment and, therefore, enable mass manufacturing of such components. The nanopowder of the present invention can be prepared using known techniques, or purchased directly at a price comparable to that that of micron-size powder. A dispersant is preferably used for reducing agglomeration of the particles which could lead to undesirable/low silver particle loading during mixing of the paste. The nanopowder of the present invention, preferably together with the dispersant, can be combined with a polymer binder that preferably has a volatilization temperature below the desired sintering temperature. Using a binder that preferably does not volatilize until close to the sintering temperature for the metal or metal alloy powder, assists in achieving denser interconnections since sintering occurs more uniformly throughout the composition (i.e., the binder is preferably chosen and formulated into the composition such that the metal or metal alloy powder on the edges closer to the source of heat does not start to fuse with neighboring particles until the bulk of the particles begins to fuse). Dispersion of the metal or metal alloy powder in the binder can be facilitated by ultrasonic or mechanical or methods, or combinations of the same. Processing conditions that allow for low temperature processing over short time periods have also been discovered. These processing conditions include both a drying and a sintering step, during each of which the temperature is rapidly ramped up. Using the processing conditions of the present invention, effective bonding can be achieved at lower temperature without the need for extended processing times. The compositions of the present invention have a wide range of applications. For example, they can be used to bond silicon integrated circuit chips in computers, or silicon power chips in power supplies, or optoelectronic chips in telecommunications modules. Also, in the case of silver powder, and silver alloys, where the metal melts at temperatures over 700° C. or 800° C., the invention is suitable for attaching semiconductor chips that can be operated at high temperatures, e.g., SiC or GaN power chips. That is, by sintering silver or silver alloy that is in the form of a nanopowder (one that is less than 500 nm in size, and most preferably below 100 nm in size) at a relatively low temperatures (e.g., on the order of 300° C., a dense, conductive metal interconnection is achieved that can be operated at high temperatures without risking melting of the interconnect, as would be the case with commercial lead and lead free solders as well as conductive epoxies. The ability to allow these chips to operate at a high temperatures cuts down their cooling requirement, leading to savings in materials and energy in the manufacture and operations of the product. The nanosilver paste of this invention, due to its high melting temperature and low processing temperature, is also useful for applications other than the attachment of silicon devices and heatsinks. It may be used to attach/interconnect wide bandgap devices that need to operate at elevated temperature such as SiC, GaN and diamond. It is also useful for attaching devices that generate substantial amounts of heat such as light-emitting diodes (LED) and semiconductor lasers. The nanosilver paste of this invention may also be formed into nanosilver preforms for easy handling and application. Nanosilver performs are formed by drying an amount of nanosilver paste on a substrate from which the preform can be easily removed. The preforms can be formed in a variety of sizes and shapes which can be easily applied and further processed when needed. The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: Continue reading about Nanoscale metal paste for interconnect and method of use... Full patent description for Nanoscale metal paste for interconnect and method of use Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Nanoscale metal paste for interconnect and method of use patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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